Planetary transmissions are used in applications where strength and durability are required. One such transmission application is in work machines used for earth moving and other heavy duty applications. The strength of the planetary transmission is provided by having multiple planet gears intermeshed with the ring gear and sun gear of each planetary gear set. The multiple planet gears allow higher torque levels to be transmitted than would be possible using single gears as there is greater contact area between the intermeshed gears. Also, since the planet gears are evenly spaced about a planet carrier, the load from the planet gears is more evenly spread around the ring gear of the planetary gear set thereby further increasing the durability of planetary gear sets as compared to other gearing configurations.
For many work machines, it is desirable to have a large number of gear ratios so that the work machine can operate over a wide range of speeds while maintaining the engine in a narrow speed range which delivers maximum torque. The simplest means to provide a large number of gear ratios using planetary gear sets is to have a planetary gear set for each of the desired gear ratios. However, there are a couple of drawbacks to using a large number of planetary gear sets. The first drawback is the high cost associated with a large number of the planetary gear sets. The second drawback is that the greater contact area between the gears of the planetary gear set results in higher frictional losses as power is transmitted through the transmission, thus, reducing the overall efficiency of the transmission.
What is needed therefore is planetary transmission which overcomes the above-mentioned drawbacks.